As a flight climbs after takeoff, the air pressure in the cabin decreases. As a result, the cabin air expands by about 30%. When the flight descends to land, the volume of the air contracts again. These changes will not normally cause any problems, as long as the gases involved – such as the air in your lungs – can move about freely. They can, however, cause discomfort or even pain if the gas is enclosed and the different pressures thus cannot be equalised.
The middle ear
The takeoff and climb to cruising altitude do not usually cause much of a problem in terms of the ears. The cabin pressure will gradually decrease to less than that of the middle ear. But the “overpressure” in the middle ear can be equalised fairly easily via the Eustachian tube.
Landing often presents more of a problem here. During an aircraft’s descent, the cabin air pressure will steadily increase; and, as it does, anyone with a cold (and the associated swelling of the mucous membranes) may be unable to adequately equalise the pressure difference between the middle ear and the ambient air via the Eustachian tube. This can result in the pressure in the middle ear being lower than that of the cabin, and this can put painful pressure on the eardrums. For further information here, see the "Earache" section.
The paranasal sinuses
The paranasal sinuses are located behind the bones of the face and the skull, and are connected to the nose and throat via small openings called ostia. The sinuses contain air, and this will expand as the aircraft climbs and contract as it descends. If you have a cold (or suffer from hayfever), the resulting swelling of the mucous membranes may block the ostia, and the resulting trapped air can cause pain or discomfort, especially during the descent. In such cases, we advise you to use a nasal spray.
Air trapped in teeth or organs
Air that is trapped in a dental filling or an abscess can also cause pain to air travellers. In these cases, the discomfort is usually more pronounced during the takeoff and climb phase. Both the stomach and the intestine also contain swallowed air and gases, and these can lead the traveller to feel bloated or suffer flatulence in some cases.